Optical add drop multiplexer

ABSTRACT

An optical add/drop multiplexer includes a mode converter. A first mode coupler is coupled to an input of the mode converter. A second mode coupler is coupled to an output of the mode converter. The mode converter includes an optical fiber with multiple cladding modes and a single core mode guided along a core. An acoustic wave propagation member is coupled to the optical fiber. The acoustic wave propagation member propagates an acoustic wave from its proximal to its distal end. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 09/426,060,filed Oct. 22, 1999, now U.S. Pat. No. 6,266,462 which is acontinuation-in-part of U.S. Ser. No. 09/022,413 filed Feb. 12, 1998,now U.S. Pat. No. 6,021,237, issued Feb. 1, 2000, both of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to telecommunication systems andassemblies, and more particularly to an optical add/drop multiplexer.

2. Description of Related Art

In modem telecommunication systems, many operations with digital signalsare performed on an optical layer. For example, digital signals areoptically amplified, multiplexed and demultiplexed. In long fibertransmission lines, the amplification function is performed by ErbiumDoped Fiber Amplifiers (EDFA's). The amplifier is able to compensate forpower loss related to signal absorption, but it is unable to correct thesignal distortion caused by linear dispersion, 4-wave mixing,polarization distortion and other propagation effects, and to get rid ofnoise accumulation along the transmission line. For these reasons, afterthe cascade of multiple amplifiers the optical signal has to beregenerated every few hundred kilometers. In practice, the regenerationis performed with electronic repeaters using optical-to-electronicconversion. However to decrease system cost and improve its reliabilityit is desirable to develop a system and a method of regeneration, orsignal refreshing, without optical to electronic conversion. An opticalrepeater that amplifies and reshapes an input pulse without convertingthe pulse into the electrical domain is disclosed, for example, in theU.S. Pat. No. 4,971,417, “Radiation-Hardened Optical Repeater”. Therepeater comprises an optical gain device and an optical thresholdingmaterial producing the output signal when the intensity of the signalexceeds a threshold. The optical thresholding material such aspolydiacetylene thereby performs a pulse shaping function. The nonlinearparameters of polydiacetylene are still under investigation, and itsability to function in an optically thresholding device has to beconfirmed.

Another function vital to the telecommunication systems currentlyperformed electronically is signal switching. The switching function isnext to be performed on the optical level, especially in the WavelengthDivision Multiplexing (WDM) systems. There are two types of opticalswitches currently under consideration. First, there are wavelengthinsensitive fiber-to-fiber switches. These switches (mechanical, thermoand electro-optical etc.) are dedicated to redirect the traffic from oneoptical fiber to another, and will be primarily used for networkrestoration and reconfiguration. For these purposes, the switching timeof about 1 msec (typical for most of these switches) is adequate;however the existing switches do not satisfy the requirements for lowcost, reliability and low insertion loss. Second, there are wavelengthsensitive switches for WDM systems. In dense WDM systems having a smallchannel separation, the optical switching is seen as a wavelengthsensitive procedure. A small fraction of the traffic carried by specificwavelength should be dropped and added at the intermediate communicationnode, with the rest of the traffic redirected to different fiberswithout optical to electronic conversion. This functionality promisessignificant cost saving in the future networks. Existing wavelengthsensitive optical switches are usually bulky, power-consuming andintroduce significant loss related to fiber-to-chip mode conversion.Mechanical switches interrupt the traffic stream during the switchingtime. Acousto-optic tunable filters, made in bulk optic or integratedoptic forms, (AOTFs) where the WDM channels are split off by coherentinteraction of the acoustic and optical fields though fast, less thanabout 1 microsecond, are polarization and temperature dependent.

Furthermore, the best AOTF consumes several watts of RF power. hasspectral resolution about 3 nm between the adjacent channels (which isnot adequate for current WDM requirements), and introduces over 5 dBloss because of fiber-to-chip mode conversions.

Another wavelength-sensitive optical switch may be implemented with atunable Fabry Perot filter (TFPF). When the filter is aligned to aspecific wavelength, it is transparent to the incoming optical power.Though the filter mirrors are almost 100% reflective no power isreflected back from the filter. With the wavelength changed or thefilter detuned (for example, by tilting the back mirror), the filterbecomes almost totally reflective. With the optical circulator in frontof the filter, the reflected power may be redirected from the incidentport. The most advanced TFPF with mirrors built into the fiber and PZTalignment actuators have only 0.8 dB loss. The disadvantage of thesefilters is a need for active feedback and a reference element forfrequency stability.

There is a need for an improved optical add/drop multiplexers andimproved narrowband AO converters that include AOTF's.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide animproved optical add/drop multiplexer.

Another object of the present invention is to provide an improvednarrowband AO converter.

These and other objects of the present invention are achieved in anoptical add/drop multiplexer that includes a mode converter. A firstmode coupler is coupled to an input of the mode converter. A second modecoupler is coupled to an output of the mode converter. The modeconverter includes an optical fiber with multiple cladding modes and asingle core mode guided along a core. An acoustic wave propagationmember is coupled to the optical fiber. The acoustic wave propagationmember propagates an acoustic wave from its proximal to its distal end.At least one acoustic wave generator is coupled to the proximal end ofthe acoustic wave propagation member.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 (a) is a schematic diagram of one embodiment of an AOTF of thepresent invention.

FIG. 1 (b) is a cross-sectional view of the optical fiber of the FIG. 1AOTF.

FIG. 2 is a cross-sectional view of one embodiment of an acoustic wavepropagation member that can be used with the AOTF of FIG. 1.

FIG. 3(a) is a cross-sectional view illustrating one embodiment of aninterface created between an optical fiber and a channel formed in anacoustic wave propagation member of the FIG. 1 AOTF.

FIG. 3(b) is a cross-sectional view illustrating an embodiment of aninterface between an optical fiber and a channel formed in an acousticwave propagation member of the FIG. 1 AOTF where a bonding material isused.

FIG. 4 is a schematic diagram of one embodiment of an AOTF of thepresent invention with an acoustic damper.

FIG. 5 is a cross-sectional view of one embodiment of an index profileof an optical fiber, useful with the AOTF of FIG. 1, that has a doublingcladding.

FIG. 6 is a cross-sectional view of an optical fiber with sections thathave different diameters.

FIG. 7 is a cross-sectional view of an optical fiber with a taperedsection.

FIG. 8 is a perspective view of one embodiment of an AOTF of the presentinvention that includes a heatsink and two mounts.

FIG. 9 is a perspective view of one embodiment of an AOTF of the presentinvention with a filter housing.

FIG. 10 is a block diagram of an optical communication system with oneor more AOTF's of the present invention.

FIG. 11 is a schematic view showing the structure of an acousto-optictunable filter according to one embodiment of the present invention.

FIG. 12 is a graph showing the coupling and transmittance of the filterof FIG. 1.

FIG. 13 is a graph showing the transmittance of the filter of FIG. 11.

FIG. 14 is a graph showing the center wavelength of filter of FIG. 1 asa function of the frequency applied to the acoustic wave generator.

FIGS. 15(a)-(d) are graphs illustrating the transmissions of the filterof FIG. 11 when multiple frequencies are applied to the acoustic wavegenerator.

FIGS. 16(a)-(b) are graphs showing the transmittance characteristics ofthe filter of FIG. 11 when varying an electric signal with a threefrequency component applied to the filter.

FIGS. 17(a)-(d) are graphs for comparing the mode convertingcharacteristic of the filter according to an embodiment of the presentinvention with that of a conventional wavelength filter.

FIG. 18(a) illustrate one embodiment of a transmission spectrum of theFIG. 11 filter.

FIG. 18(b) illustrates the measured and the calculated centerwavelengths of the notches as a function of acoustic frequency of anembodiment of the FIG. 11 filter.

FIG. 19 illustrates two examples of configurable spectral profiles withspectral tilt from the FIG. 11 filter.

FIG. 20(a) is a filter assembly that includes two filters of FIG. 11that are in series.

FIG. 20(b) is a schematic diagram of a dual-stage EDFA with a filter ofFIG. 20(a).

FIG. 21 (a) is a graph of gain profiles of an EDFA with the filter ofFIG. 20(a).

FIG. 21 (b) is a graph illustrating filter profiles that produced theflat gain profiles shown in FIG. 21(a).

FIG. 21 (c) is a graph illustrating filter profiles of the FIG. 20(a)filter assembly.

FIGS. 22(a) and 22(b) are graphs illustrating the polarizationdependence of one embodiment of the filter of the present invention.

FIG. 23 illustrates one embodiment of the present invention from FIG. 4that has a reduction with a lower polarization dependent loss.

FIGS. 24(a) and 24(b) are graphs illustrating the polarization dependentloss profile of one embodiment of the invention, from FIG. 4, when thefilter is operated to produce 10-dB attenuation at 1550 nm.

FIG. 25 illustrates an embodiment of the invention with two of thefilters of FIG. 1.

FIG. 26 is a graph illustrating the effects of a backward acousticreflection at the damper of one embodiment of the present invention fromFIG. 4.

FIG. 27(a) is a graph illustrating, in one embodiment of FIG. 4, themodulation depth at 10-dB attenuation level at both first- andsecond-harmonics of the acoustic frequency.

FIG. 27(b) is a graph illustrating the modulation depth of first- andsecond-harmonics components from FIG. 27(a).

FIG. 28 is a schematic diagram of one embodiment of an optical add/dropmultiplexer of the present invention.

FIG. 29 is a schematic diagram of a second embodiment of an opticaladd/drop multiplexer of the present invention.

FIG. 30 is a schematic diagram of a narrowband AO converter of thepresent invention.

FIG. 31 is a schematic diagram of an interleaver embodiment of thepresent invention using the add/drop multiplexers of FIG. 29.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of an AOTF (hereafter filter 10) ofthe present invention. A non-birefringent single mode optical fiber 12has a longitudinal axis, a core 14 and a cladding 16 in a surroundingrelationship to core 14. Optical fiber 12 has multiple cladding modesand a single core mode guided along core 14. Optical fiber 12 providesfundamental and cladding mode propagation along a selected length ofoptical fiber 12. Alternatively, optical fiber 12 does not have multiplecladding modes and a single core mode, and it can a birefringent singlemode fiber. In one embodiment, optical fiber 12 is tensioned. Sufficienttensioning can be applied in order to reduce losses in a flexure wavepropagated in optical fiber 12.

The core of the non-birefringent fiber is substantiallycircular-symmetric. The circular symmetry ensures that the refractiveindex of the core mode is essentially insensitive to the state ofoptical polarization. In contrast, in hi-birefringent single mode fibersthe effective refractive index of the core mode is substantiallydifferent between two principal polarization states. The effectiverefractive index difference between polarization modes in highbirefringence single mode fibers is generally greater than 10⁻⁴. Ahighly elliptical core and stress-inducing members in the claddingregion are two main techniques to induce large birefringence. Innon-birefringent fibers, the effective index difference betweenpolarization states is generally smaller than 10⁻⁵.

An acoustic wave propagation member 18 has a distal end 20 that iscoupled to optical fiber 12. Acoustic wave propagation member 18propagates an acoustic wave from a proximal end 22 to distal end 20 andlaunches a flexural wave in optical fiber 12. The flexural wave createsa periodic microbend structure in the optical fiber. The periodicmicrobend induces an antisymmetric refractive index change in the fiberand, thereby, couples light in the fiber from a core mode to claddingmodes. For efficient mode coupling, the period of the microbending, orthe acoustic wavelength, should match the beatlength between the coupledmodes. The beatlength is defined by the optical wavelength divided bythe effective refractive index difference between the two modes.

Acoustic wave propagation member 18 can be mechanically coupled to theoptical fiber and minimizes acoustic coupling losses in between theoptical fiber and the acoustic wave propagation member. In oneembodiment, acoustic wave propagation member 18 is coupled to opticalfiber 12 in a manner to create a lower order mode flexure wave inoptical fiber 12. In another embodiment, acoustic wave propagationmember 18 is coupled to the optical fiber to match a generation of modescarried by optical fiber 12.

Acoustic wave propagation member 18 can have a variety of differentgeometric configurations but is preferably elongated. In variousembodiments, acoustic wave propagation member 18 is tapered proximal end22 to distal end 20 and can be conical. Generally, acoustic wavepropagation member 18 has a longitudinal axis that is parallel to alongitudinal axis of optical fiber 12.

At least one acoustic wave generator 24 is coupled to proximal end 22 ofacoustic wave propagation member. Acoustic wave generator 24 can be ashear transducer.

Acoustic wave generator 24 produces multiple acoustic signals withindividual controllable strengths and frequencies. Each of the acousticsignals can provide a coupling between the core mode and a differentcladding mode. Acoustic wave generator 24 can produce multiple acousticsignals with individual controllable strengths and frequencies. Each ofthe acoustic signals provides a coupling between the core mode and adifferent cladding mode of optical fiber 12. A wavelength of an opticalsignal coupled to cladding 16 from core 14 is changed by varying thefrequency of a signal applied the acoustic wave generator 24.

Acoustic wave generator 24 can be made at least partially of apiezoelectric material whose physical size is changed in response to anapplied electric voltage. Suitable piezoelectric materials include butare not limited to quartz, lithium niobate and PZT, a composite of lead,zinconate and titanate. Other suitable materials include but are notlimited to zinc monoxide. Acoustic wave generator 24 can have amechanical resonance at a frequency in the range of 1-20 MHz and becoupled to an RF signal generator.

Referring now to FIG. 2, one embodiment of acoustic wave propagationmember 18 has an interior with an optical fiber receiving channel 26.Channel 26 can be a capillary channel with an outer diameter slightlygreater than the outer diameter of the fiber used and typically in therange of 80˜150 microns. The length of the capillary channel ispreferably in the range of 5˜15 mm. The interior of acoustic wavepropagation member 18 can be solid. Additionally, acoustic wavepropagation member 18 can be a unitary structure.

Optical fiber 12 is coupled to acoustic wave propagation member 18. Asillustrated in FIG. 3(a), the dimensions of channel 26 and an outerdiameter of optical fiber 12 are sufficiently matched to place the twoin a contacting relationship at their interface. In this embodiment, therelative sizes of optical fiber 12 and channel 26 need only besubstantially the same at the interface. Further, in this embodiment,the difference in the diameter of optical fiber 12 and channel 26 are inthe range of 1˜10 microns.

In another embodiment, illustrated in FIG. 3(b), a coupling member 28 ispositioned between optical fiber 12 and channel 26 at the interface.Suitable coupling members 28 including but are not limited to bondingmaterials, epoxy, glass solder, metal solder and the like.

The interface between channel 26 and optical fiber 12 is mechanicallyrigid for efficient transduction of the acoustic wave from the acousticwave propagation member 18 to the optical fiber 12.

Preferably, the interface between optical fiber 12 and channel 26 issufficiently rigid to minimize back reflections of acoustic waves fromoptical fiber 12 to acoustic wave propagation member 18.

In the embodiments of FIGS. 3(a) and 3(b), acoustic wave propagationmember 18 is a horn that delivers the vibration motion of acoustic wavegenerator 24 to optical fiber 12. The conical shape of acoustic wavepropagation member 18, as well as its focusing effect, providesmagnification of the acoustic amplitude at distal end 20, which is asharp tip. Acoustic wave propagation member 18 can be made from a glasscapillary, such as fused silica, a cylindrical rod with a central hole,and the like.

In one embodiment, a glass capillary is machined to form a cone and aflat bottom of the cone was bonded to a PZT acoustic wave generator 24.Optical fiber 12 was bonded to channel 26. Preferably, distal end 20 ofacoustic wave generator 18 is as sharp as possible to minimizereflection of acoustic waves and to maximize acoustic transmission.Additionally, the exterior surface of acoustic wave generator 18 issmooth. In another embodiment, acoustic wave generator 18 is a horn witha diameter that decreases exponentially from proximal end 22 to distalend 20.

As illustrated in FIG. 4, filter 10 can also include an acoustic damper30 that is coupled to optical fiber 12. Acoustic damper 30 includes ajacket 32 that is positioned in a surrounding relationship to opticalfiber 12. Acoustic damper 30 absorbs incoming acoustic waves andminimizes reflections of the acoustic wave. The reflected acoustic wavecauses an intensity modulation of the optical signal passing through thefilter by generating frequency sidebands in the optical signal. Theintensity modulation is a problem in most applications. A proximal end34 of the acoustic damper 30 can be tapered. Acoustic damper 30 can bemade of a variety of materials. In one embodiment, acoustic damper 30 ismade of a soft material that has a low acoustic impedance so thatminimizes the reflection of the acoustic wave. Jacket 32 itself is asatisfactory damper and in another embodiment jacket 32 takes the placeof acoustic damper 30. Optionally, jacket 32 is removed from thatportion of optical fiber 12 in a interactive region 36 and that portionof optical fiber 12 that is bonded to acoustic wave generator 24.

The interactive region is where an optical signal is coupled to cladding16 from core 14. This coupling is changed by varying the frequency of asignal applied to acoustic wave generator 24. In one embodiment,interactive region 36 extends from distal end 20 to at least a proximalportion within acoustic damper 30. In another embodiment, interactiveregion 36 extends from distal end 20 and terminates at a proximal end ofacoustic damper 30. In one embodiment, the length of optical fiber 12 ininteractive region is less than 1 meter, and preferably less than 20 cm.The nonuniformity of the fiber reduces the coupling efficiency and alsocauses large spectral sidebands in the transmission spectrum of thefilter. Another problem of the long length is due to the modeinstability. Both the polarization states of the core and cladding modesand the orientation of the symmetry axis of an antisymmetric claddingmode are not preserved as the light propagates over a long lengthgreater than 1 m. This modal instability also reduces the couplingefficiency and causes large spectral sidebands. Preferably, the outerdiameter of optical fiber 12, with jacket 32, is in the range of 60-150microns.

The profile of the refractive index of the cross section of opticalfiber 12 influences its filtering characteristics. One embodiment ofoptical fiber 12, illustrated in FIG. 5, has a first and second cladding16′ and 16″ with core 14 that has the highest refractive index at thecenter. First cladding 16′ has an intermediate index and second cladding16″ has the lowest index. Most of the optical energy of severallowest-order cladding modes is confined both only in core 14 and firstcladding 16′. The optical energy falls exponentially from the boundarybetween first and second claddings 16′ and 16″, respectively.

Optical fields are negligible at the interface between second cladding16″ and the surrounding air, the birefringence in the cladding modes,due to polarization-induced charges, is much smaller than inconventional step-index fibers where second cladding 16″ does not exist.The outer diameter of first cladding 16′ is preferably smaller than thatof second cladding 16″, and can be smaller by at least 5 microns. In onespecific embodiment, core 14 is 8.5 microns, first cladding 16′ has anouter diameter of 100 microns and second cladding 16″ has an outerdiameter of 125 microns. Preferably, the index difference between core14 and first cladding 16′ is about 0.45%, and the index differencebetween first and second claddings 16′ and 16″ is about 0.45%.

In another embodiment, the outer diameter of first cladding 16′ issufficiently small so that only one or a few cladding modes can beconfined in first cladding 16′. One specific example of such an opticalfiber 12 has a core 14 diameter of 4.5 microns, first cladding 16′ of 10microns and second cladding 16″ of 80 microns, with the index differencebetween steps of about 0.45% each.

The optical and acoustic properties of optical fiber 12 can be changedby a variety of different methods including but not limited to, (i)fiber tapering, (ii) ultraviolet light exposure, (iii) thermal stressannealing and (iv) fiber etching.

One method of tapering optical fiber 12 is achieved by heating andpulling it. A illustration of tapered optical fiber 12 is illustrated inFIG. 6. As shown, a uniform section 38 of narrower diameter is createdand can be prepared by a variety of methods including but not limited touse of a traveling torch. Propagation constants of optical modes can begreatly changed by the diameter change of optical fiber 12. The pullingprocess changes the diameter of core 14 and cladding 16 and also changesthe relative core 14 size due to dopant diffusion. Additionally, theinternal stress distribution is modified by stress annealing. Taperingoptical fiber 12 also changes the acoustic velocity.

When certain doping materials of optical fiber 12 are exposed toultraviolet light their refractive indices are changed. In oneembodiment, Ge is used as a doping material in core 14 to increase theindex higher than a pure SiO₂ cladding 16. When a Ge-doped optical fiber12 is exposed to ultraviolet light the index of core 14 can be changedas much as 0.1%. This process also modifies the internal stress fieldand in turn modifies the refractive index profile depending on theoptical polarization state. As a result, the birefringence is changedand the amount of changes depends on optical modes. This results inchanges of not only the filtered wavelength at a given acousticfrequency or vice versa but also the polarization dependence of thefilter. Therefore, the UV exposure can be an effective way of trimmingthe operating acoustic frequency for a given filtering wavelength aswell as the polarization dependence that should preferably be as smallas possible in most applications.

Optical fiber 12 can be heated to a temperature of 800 to 1,300° C. orhigher to change the internal stresses inside optical fiber 12. Thisresults in modification of the refractive index profile. The heattreatment is another way of controlling the operating acoustic frequencyfor a given filtering wavelength as well as the polarization dependence.

The propagation velocity of the acoustic wave can be changed bychemically etching cladding 16 of optical fiber 12. In this case, thesize of core 14 remains constant unless cladding is completely etched.Therefore, the optical property of core mode largely remains the same,however, that of a cladding mode is altered by a different claddingdiameter. Appropriate etchants include but are not limited to hydrofluoride (HF) acid and BOE.

The phase matching of optical fiber 12 can be chirped. As illustrated inFIG. 6, a section 40 of optical fiber can have an outer diameter thatchanges along its longitudinal length. With section 40, both the phasematching condition and the coupling strength are varied along its z-axis42 and the phase matching conditions for different wavelengths satisfiedat different positions along the axis. The coupling then can take placeover a wide wavelength range. By controlling the outer diameter as afunction of its longitudinal axis 42, one can design varioustransmission spectrum of the filter. For example, uniform attenuationover a broad wavelength range is possible by an appropriate diametercontrol.

Chirping can also be achieved when the refractive index of core 14 isgradually changed along z-axis 42. In one embodiment, the refractiveindex of core 14 is changed by exposing core 14 to ultraviolet lightwith an exposure time or intensity as a function of position along thelongitudinal axis. As a result, the phase matching condition is variedalong z-axis 42. Therefore, various shapes of transmission spectrum ofthe filter can be obtained by controlling the variation of therefractive index as a function of the longitudinal axis 42.

As illustrated in FIG. 8 a heatsink 44 can be included to cool acousticwave generator. In one embodiment, heatsink 44 has a proximal face 46and a distal face 48 that is coupled to the acoustic wave generator 24.Preferably, acoustic wave generator 24 is bonded to distal face 48 byusing a low-temperature-melting metal-alloy solder including but notlimited to a combination of 95% zinc and 5% tin and indium-based soldermaterials. Other bonding material includes heat curable silver epoxy.The bonding material should preferably provide good heat and electricalconduction. Heatsink 44 provides a mount for the acoustic wave generator24. Heatsink can be made of a variety of materials including but notlimited to aluminum, but preferably is made of a material with a highheat conductivity and a low acoustic impedance.

Acoustic reflections at proximal face can be advantageous if controlled.By introducing some amount of reflection, and choosing a right thicknessof heatsink 44, the RF response spectrum of acoustic wave generator 24can be modified so the overall launching efficiency of the acoustic wavein optical fiber can be less dependent on the RF frequency.

In this case, the reflectivity and size of heatsink 44 is selected toprovide a launching efficiency of the flexural wave into optical fiber12 almost independent of an RF frequency applied to acoustic wavegenerator 24. The thickness of heatsink 44 is selected to provide atravel time of an acoustic wave from distal face 48 to proximal face 46,and from proximal face 46 to distal face 48 that substantially matches atravel time of the acoustic wave traveling through acoustic wavepropagation member 24 from its proximal end to its distal end, and fromits distal end to its proximal end. The heat sink material or thematerial for the attachment to the proximal face 46 is selected toprovide the amount of back reflection from the heat sink thatsubstantially matches the amount of back reflection from the acousticwave propagation member. In various embodiments, the proximal and distalfaces, 46, 48 of heatsink 44 have either rectangular or circular shapeswith the following dimensions: 10×10 mm² for the rectangular shape anddiameter of 10 mm for the cylindrical shaped heat sink.

However, acoustic back reflections due to proximal face 46 arepreferably avoided. Acoustic reflections from the heat sink back to theacoustic wave generator are reduced by angling proximal face 46 at anangle greater than 45 degree or by roughing the face. The acoustic wavecoming from the acoustic generator toward the angled proximal face 46 isreflected away from the acoustic generator, reducing the acoustic backreflection to the acoustic wave generator. The roughed face also reducesthe acoustic reflection by scattering the acoustic wave to randomdirections. Preferably, the side faces of the heat sink are alsoroughened or grooved to scatter the acoustic wave and thereby to avoidthe acoustic back reflection. Another method to reduce the backreflection is to attach an acoustic damping material at the proximalface 46. Suitable materials that reduce back reflections include softpolymers, silicone, and the like that can be applied to proximal face46.

Referring again to FIG. 8, an acoustic damper mount 50 supports acousticdamper 30. Acoustic damper mount 50 can be made of a variety ofmaterials including but not limited to silica, invar, and the like. Afilter mount 52 supports heatsink 44 and acoustic damper mount 50. Inone embodiment, filter mount is a plate-like structure. Preferably,filter mount 52 and optical fiber 12 have substantially the same thermalexpansion coefficients. Filter mount 52 and fiber 12 can be made of thesame materials.

Filter mount 52 and optical fiber 12 can have different thermalexpansion coefficients and be made of different materials. In oneembodiment, filter mount 52 has a lower thermal expansion coefficientthan optical fiber 12. Optical fiber 12 is tensioned when mounted andbonded to the filter mount 52. The initial strain on optical fiber 12 isreleased when the temperate increases because the length of filter mount52 is increased less than optical fiber 12. On the other hand, when thetemperature decreases optical fiber 12 is stretched further. When theamount of strain change according to temperature change is appropriatelychosen by selecting proper material for mount the 52, the filteringwavelength of filter 10 can be made almost independent of temperature.Without such mounting arrangement, the center wavelength of the filterincreases with temperature. Additionally, interactive region 36 of issufficiently tensioned to compensate for changes in temperature of theinteractive region 36 and filter mount 52.

In another embodiment, illustrated in FIG. 9, a filter housing 54encloses interactive region 36. Filter housing 54 can be made of avariety of materials, including but not limited to silica, invar and thelike. Filter housing 54 eliminates the need for a separate filter mount52. Filter housing 54 extends from distal face 48 of heatsink 44 toacoustic damper 30 or to a jacketed portion 32 of optical fiber 12.Acoustic wave propagation member 18, acoustic wave generator 24 and theacoustic damper 30 can be totally or at least partially positioned in aninterior of filter housing 54.

In one embodiment, filter housing 54 and optical fiber 12 are made ofmaterials with substantially similar thermal expansion coefficients. Asuitable material is silica. Other materials are also suitable andinclude invar. Filter housing 54 and optical fiber 12 can have differentthermal expansion coefficients and be made of different materials. Inone embodiment, filter housing 54 has a lower thermal expansioncoefficient than optical fiber 12.

In one embodiment, interactive region 36 is sufficiently tensionedsufficiently to compensate for changes in temperature of interactiveregion 36 and filter housing 54.

As illustrated in FIG. 10, filter 10 can be a component or subassemblyof an optical communication system 56 that includes a transmitter 58 anda receiver 60. Transmission 58 can include a power amplifier with filter10 and receiver 60 can also include a pre amplifier that includes filter10. Additionally, optical communication system 56 may also have one ormore line amplifiers that include filters 10.

Referring now to FIG. 11, if an electric signal 57 with constantfrequency “f” is applied to acoustic wave generator 24, a flexuralacoustic wave having the same frequency “f” is generated. The flexuralacoustic wave is transferred to optical fiber 12 and propagates alongoptical fiber 12, finally absorbed in acoustic damper 30. The flexuralacoustic wave propagating along optical fiber 12 produces periodicmicrobending along the fiber, resulting in the periodic change ofeffective refractive index which the optical wave propagating alongoptical fiber 12 experiences. The signal light propagating along opticalfiber 12 in a core mode can be converted to a cladding mode by thechange of effective refractive index in optical fiber 12.

When signal light is introduced into filter 10 part of the signal lightis converted to a cladding mode due to the effect of the acoustic waveand the remainder of the signal light propagates as a core mode whilethe signal light propagates along interactive region 36. The signallight converted to a cladding mode cannot propagate any longer inoptical fiber 12 with jacket 32 because the light is partly absorbed inoptical fiber 12 or partly leaks from optical fiber 12. A variety ofmode selecting means, including a mode conversion means between coremodes and cladding modes, can be incorporated in filter 10. For example,the long-period grating described in the article “Long-periodfiber-grating based gain equalizers” by A. M. Vengsarkar et al. inOptics Letters, Vol. 21, No. 5, p. 336, 1996 can be used as the modeselecting means. As another example, a mode coupler, which converts oneor more cladding modes of one fiber to core modes of the same fiber oranother fiber, can also be used.

A flexural acoustic wave generated by acoustic wave propagation member18 propagates along interactive region 36. The acoustic wave createsantisymmetric microbends that travel along interactive region 36,introducing a periodic refractive-index perturbation along optical fiber12. The perturbation produces coupling of an input symmetric fundamentalmode to an antisymmetric cladding mode when the phase-matching conditionis satisfied in that the acoustic wavelength is the same as the beatlength between the two modes. The coupled light in the cladding mode isattenuated in jacket 32. For a given acoustic frequency, the couplingbetween the fundamental mode and one of the cladding modes takes placefor a particular optical wavelength, because the beat length hasconsiderable wavelength dispersion. Therefore, filter 10 can be operatedas an optical notch filter. A center wavelength and the rejectionefficiency are tunable by adjustment of the frequency and the voltage ofRF signal applied to acoustic wave propagation member 18, respectively.

The coupling amount converted to a cladding mode is dependent on thewavelength of the input signal light. FIG. 12(a) shows the couplingamounts as functions of wavelength when flexural acoustic waves at thesame frequency but with different amplitudes are induced in opticalfiber 12. As shown in FIG. 12(a), the coupling amounts are symmetricalwith same specific wavelength line (λ_(c)), i.e., center wavelengthline, however they show different results 62 and 64 due to the amplitudedifference of the flexural acoustic waves. Therefore, the transmittanceof the output light which has passed through filter 12 is differentdepending on the wavelength of the input light. Filter 12 can act as anotch filter which filters out input light with specific wavelength asshown in FIG. 12(b).

FIG. 12(b) is a graph showing the transmittances as a function ofwavelength when flexural acoustic waves with different amplitudes areinduced in filter 12. The respective transmittances have same centerwavelength as does the coupling amount, but different transmittancecharacteristic 64 and 66 depending on the amplitude difference of theflexural acoustic waves can be shown.

The center wavelength λ_(c), of filter 10 satisfies the followingequation.

β_(co)(λ)−β_(cl)(λ)=2π/λ_(a)

In the above equation, β_(co)(λ) and β_(cl)(λ) are propagation constantsof core mode and cladding mode in optical fiber 12 which arerespectively dependent on the wavelength, and λ_(a) represents thewavelength of the flexural acoustic waves.

Accordingly, if the frequency of the electric signal applied to acousticwave generator 24 varies, the wavelength of the acoustic wave generatedin optical fiber 12 also varies, which results in the center wavelengthchange of filter 10. In addition, since the transmission is dependent onthe amplitude of the flexural acoustic wave, the transmission of signallight can be adjusted by varying the amplitude of the electric signalwhich is applied to acoustic wave generator 24.

FIG. 13 is a graph showing the transmittance of filter 10 in oneembodiment when different electric signal frequencies are applied. Asshown in FIG. 13, each center wavelength (i.e., wavelength showingmaximum attenuation) of filter 10 for different electric signals was1530 nm, 1550 nm and 1570 nm. Therefore the center wavelength of filter10, according to the embodiment, is changed by varying the frequency ofthe electric signal which is applied to acoustic wave generator 24.

As described above, since there are a plurality of cladding modes ininteractive region 36 the core mode can be coupled to several claddingmodes. FIG. 14 is a graph showing the center wavelength of filter 10according to the embodiment of the invention as a function of thefrequency applied to the flexural acoustic wave generator. In FIG. 14,straight lines 71, 72 and 73 represent the center wavelength of filter10 resulting from the coupling of a core mode with three differentcladding modes.

Referring to FIG. 14, there are three applied frequencies for any oneoptical wavelength in this case. Therefore the input signal light isconverted to a plurality of cladding modes by applying multi-frequencyelectric signal to acoustic wave generator 24. Moreover, it meanstransmission characteristics of filter 10 can be electrically controlledby adjusting the amplitude and each frequency component of the electricsignal.

As shown in FIG. 15(a), the respective transmission features 74, 75 and76 of filter 10 can be provided by applied electric signals withdifferent frequencies f1, f2 and f3. In this example, assuming that f1couples the core mode of input signal light to a cladding mode (claddingmode A), f2 couples the core mode to other cladding mode (cladding modeB) and f3 couples the core mode to another cladding mode different fromA or B (cladding mode C, the transmission feature is shown in FIG. 15(b)as a curve 77 when electric signal with three frequency components f1,f2 and S is applied to acoustic wave generator 24.

As shown in FIG. 5(c), if filter 10 has transmission feature curves 78,79 and 80 corresponding to respective frequencies f1′, f2′ and f3′ andelectric signal having three frequency components f1′, f2′ and f3′ isapplied to the flexural acoustic wave generator, the transmissionfeature of filter 10 is shown as a curve 81 of FIG. 5(d).

FIGS. 16(a) and 16(b) are graphs showing the transmittance of filter 10according to an embodiment of the present invention, when varyingelectric signal having three frequency components is applied to filter10. When varying electric signal having a plurality of frequencycomponents is applied to acoustic wave generator 24 various shapes oftransmittance curves 82, 83 and 84 can be obtained.

Since conventional tunable wavelength filters utilize the coupling ofonly two modes, the difference between a plurality of appliedfrequencies naturally becomes small to obtain wide wavelength bandfiltering feature by applying a plurality of frequencies. In this case,as described under the article “Interchannel Interference inmultiwavelength operation of integrated acousto-optical filters andswitches” by F. Tian and H. Herman in Journal of Light wave technology1995, Vol. 13, n 6, pp. 1146-1154, when signal light input to a filteris simultaneously converted into same (polarization) mode by variousapplied frequency components, the output signal light may undesirably bemodulated with frequency corresponding to the difference between theapplied frequency components. However, with filter 10 the above problemcan be circumvented, because the respective frequency components convertthe mode of input light into different cladding modes in filter 10.

In one embodiment, the filtering feature shown in FIG. 17(a) wasobtained by applying adjacent frequencies 2.239 MHz and 2.220 MHz toreproduce the result of a conventional method. The applied twofrequencies were such that convert the mode of input light into the samecladding mode. Under the condition, narrow wavelength-band signal lightwith a center wavelength of 1547 nm was input to filter 10 to measureoutput light. Referring to the measurement result shown in FIG. 17(b),there is an undesirable modulated signal with frequency corresponding tothe difference of the two applied frequencies.

In another embodiment, when adjacent frequencies 2.239 MHz and 2.220 MHzwere applied to acoustic wave generator 24, according to the embodimentof the invention, the two frequency components convert the mode of inputlight into mutually different cladding modes. FIG. 17(c) shows themeasurement result when the same signal light as the above experimentwas input to filter 10 and output light was measured.

However, an undesirable modulated signal, which appeared in aconventional filter, practically disappeared as shown in FIG. 17(d).

In optical communications or optical fiber-sensor systems, wavelengthfilters are required that has a wide tuning range and are capable ofelectrically controlling its filtering feature.

FIG. 18(a) illustrates one embodiment of a transmission spectrum offilter 10 with a 15.5-cm-long interaction length for a broadbandunpolarized input light from a LED. A conventional communication fiberwas used with a nominal core diameter of 8.5 μm, a cladding outerdiameter of 125 μm and a normalized index difference of 0.37%. Thefrequency of the applied RF signal was 2.33 MHz, and the correspondingacoustic wavelength was estimated to be ˜650 μm. The three notches shownin FIG. 8(a) are from the coupling to three different cladding modeswith the same beat length at the corresponding wavelengths. The coupledcladding modes were the LP₁₁ ^((cl)), the LP₁₂ ^((cl)), and the LP₁₃^((cl)) modes, which was confirmed from far-field radiation patterns.The center of each coupling wavelength was tunable over >100 nm bytuning the acoustic frequency.

FIG. 18(b) shows the measured and the calculated center wavelengths ofthe notches as a function of acoustic frequency. The fiber parametersused in the calculation for best fit with the experimental results are acore diameter of 8.82 μm, a cladding outer diameter of 125 μm, and anormalized index difference of 0.324%, in reasonable agreement with theexperimental fiber parameters.

Referring again to FIG. 8(a), coupling light of a given wavelength fromthe fundamental mode to different cladding modes requires acousticfrequencies that are separated from each other by a few hundredkilohertz. This separation is large enough to provide a widewavelength-tuning range of almost 50 nm for each coupling mode pairwithout significant overlap with each other, thereby practicallyeliminating the coherent cross talk that is present in conventionalcounterparts. The tuning range is sufficient to cover the bandwidth oftypical EDFA's. In one embodiment, filter 10 provides for a combinationof independent tunable notch filters built into one device, and thenumber of involved cladding modes corresponds to the number of filters.The multifrequency acoustic signals can be generated by a singletransducer, and the spectral profile of filter 10 is determined by thefrequencies and amplitudes of the multiple acoustic signals.

FIG. 19 shows two examples of the configurable spectral profiles withspectral tilt, which can be used to recover the gain flatness in an EDFAwith a gain tilt caused by signal saturation. In one embodiment, threecladding modes [LP₁₁ ^((cl )), the LP₁₂ ^((cl)), and the LP₁₃ ^((cl))]were used and three RF signals were simultaneously applied withdifferent voltages and frequencies adjusted for the particular profile.The 3-dB bandwidth of the individual notch was ˜6 nm with a 10-cm-longinteraction length.

A complex filter profile is required to flatten an uneven EDFA gain,which exhibits large peaks with different widths around 1530 and 1560nm. The combination of three Gaussian shaped passive filters can producea flat gain over a 30-nm wavelength range.

As illustrated in FIG. 20(a), a filter assembly 12 of the presentinvention can include first and second filters 10′ and 10″ in series.Each filter 10′ and 10″ is driven by three radio frequency (RF) signalsat different frequencies and amplitudes that produce acousto-optic modeconversion from the fundamental mode to different cladding modes. Thisapproach eliminates the detrimental coherent crosstalk present inLiNbO₃-based AOTF's . The 3-dB bandwidths of the first filter 10′ were3.3, 4. 1, and 4.9 nm for the couplings to the cladding modes LP₁₂^((cl)), the LP₁₃ ^((cl)), and LP₁₄ ^((cl)), respectively. For secondfilter 10″, they were 8, 8.6, and 14.5 nm for the couplings to thecladding modes, LP₁₁ ^((cl)), the LP₁₂ ^((cl)), and the LP₁₃ ^((cl))respectively.

The minimum separations of notches produced by single RF drivingfrequency were ˜50 nm for first filter 10′ and ˜150 nm for second filter10″ , respectively, so that only one notch for each driving frequencyfalls into the gain-flattening range (35 nm). The large differencebetween filters 10′ and 10″ was due to the difference in optical fiber12 outer diameters. The polarization splitting in the center wavelengthof the notches as ˜0.2 nm for first filter 10′ and ˜1.5 nm for secondfilter 10″. The relatively large polarization dependence in secondfilter 10″ is due mainly due to the unwanted core elliptically andresidual thermal stress in optical fiber 12, that can be reduced to anegligible level by using a proper optical fiber. First and secondfilters 10′ and 10″ were used for the control of the EDFA gain shapearound the wavelengths of 1530 and 1555 nm, respectively. The backgroundloss of the gain flattening AOTF was less than 0.5 dB, which was mainlydue to splicing of different single-mode fibers used in the two AOTF's1010. Adjusting the frequencies and voltages of the applied RF signalsprovided control of the positions and depths of the notches with greatflexibility. The RF's were in the range between 1 and 3 MHz.

FIG. 20(b) shows a schematic of a dual-stage EDFA employing gainflattening filter 10 along with a test setup. A 10-m-long EDF pumped bya 980-nm laser diode and a 24-m-long EDF pumped by a 1480-nm laser diodewere used as the first and the second stage amplifiers, respectively.The peak absorption coefficients of both EDF's were ˜2.5 dB/m at 1530nm. Filter 10 was inserted between the two stages along with anisolator. Total insertion loss of filter 10 and the isolator was lessthan 0.9 dB. Six synthesizers and two RF power amplifiers were used todrive filter 10.

Gain profiles of the EDFA were measured using a saturating signal at thewavelength of 1547.4 nm and a broad-band light-emitting diode (LED)probing signal. The saturating signal from a distributed feedback (DFB)laser diode was launched into the EDFA after passing through aFabry-Perot filter (optical bandwidth: 3 GHz, extinction ratio: 27 dB)to suppress the sidelobes of the laser diode. The total power of theprobe signal in 1520-1570-nm range was 27 dBm, which is much smallerthan that of the input saturating signal ranging from 13 to 7 dBm.

FIG. 21 (a) shows gain profiles before and after the gain flattening fortwo different saturating signal powers of 13 and 7 dBm when thesecond-stage pump power was 42 mW. The gain excursions before flatteningwere larger than 5 dB. By adjusting the filter profile, flat gainprofiles within 0.7 dB were obtained over 35 nm for both cases. The flatgain region is shifted slightly toward the shorter wavelength for highergain level, which is due to the intrinsic gain characteristics of theEDF. FIG. 21(b) shows filter profiles that produced the flat gainprofiles shown in FIG. 21(a), where Profile 1 and Profile 2 are for-thecases of saturating tones of 13 and 7 dBm, respectively. For themeasurements, EDFI was used as an ASE source, while the second pumpdiode (1480 nm) was turned off. The ASE signal leaked out of the secondWDM coupler was monitored and the signals obtained when the filter wason and off were compared to yield the filter response. The attenuationcoefficients for Profile 1 and Profile 2 at the saturating signalwavelength were 5.0 and 4.9 dB, respectively, and the averageattenuation over the 35-nm range (1528-1563 nm) was 5 dB in both cases.The total RF electrical power consumption of the filter was less than500 mW. Profile 2 could be obtained from Profile 1 by adjusting mainlythe depths of notches, although fine adjustments of center wavelengthsof notches within 0.5-nm range slightly improved the gain flatness. FIG.21(c) shows the filter profiles of first filter 10′ and second filter10″ used to form Profile 1, and also the locations of center wavelengthsof six notches. By adjusting first and second filters 10′ and 10″spectral profiles electronically gain flatness of <0.7 dB over 35-nmwavelength range were obtained at various levels of gain as well asinput signal and pump power.

One important characteristic of filter 10 is polarization dependence.The shape of filter 10 can be dependent on the polarization state ofinput light. The polarization dependence originates from fiberbirefringence. Fiber birefringence causes the effective propagationconstant of a mode to be different between two eigen polarizationstates. Since the magnitude of birefringence is different from mode tomode, the fiber birefringence causes the beat length between two coupledmodes to be different between the eigen polarization states, and,therefore, results in splitting of center wavelength of filter 10 for agiven acoustic frequency.

FIG. 22(a) illustrates the polarization dependence. Curve 90 representsthe filter profile for light in one eigen polarization state, and filterprofile 92 is when the input is in the other eigen state. The centerwavelengths are split because of the birefringence. Moreover, since thefield overlap between two coupled modes is also polarization dependentdue to the birefringence, the attenuation depth can be different betweenfilter profiles 90 and 92.

A critical feature due to the polarization dependence is thepolarization dependent loss (PDL) which is defined as the difference ofthe magnitude of attenuation between two eigen polarization states.Since polarization dependent loss is an absolute value, it increaseswith the attenuation depth. FIG. 22(b) shows polarization dependent lossprofile 93 associated with filter profiles 90 and 92. In WDMcommunication system applications, the polarization dependent lossshould be minimized. Most applications require the polarizationdependent loss to be less than 0.1 dB. However, acousto-optic tunablefilter 10 has exhibited a typical polarization dependent loss as largeas 2 dB at 10-dB attenuation level. This is due to the largebirefringence of the antisymmetric cladding modes.

FIG. 23 shows one possible configuration that can reduce the inherentpolarization dependent loss of filter 10. In FIG. 23, double-pass filter100 consists of a 3-port circulator with input-, middle-, andoutput-port fibers, 12′, 12″ and 12″, respectively, and Faraday rotatingmirror (FRM) 104. The middle-port fiber 12″ is connected toacousto-optic tunable filter 10 and Faraday rotating mirror 104. Whenlight comes in through input-port fiber 12′, it is directed to filter10, through circulator 102, and then refracted by Faraday rotatingmirror 104. Faraday rotating mirror 104 acts as a conjugate mirror withrespect to optical polarization states. So, if the light pass throughfilter 10 in a specific polarization state, then on the way back afterreflection it pass through filter 10 in its orthogonal polarizationstate. Since the light pass through filter 10 twice but in mutuallyorthogonal states, the total attenuation after the double pass becomespolarization-insensitive. Another benefit of the double passconfiguration is that, since the filtering takes place twice in filter10, the drive RF power applied to filter 10 to obtain a certainattenuation depth is reduced half compared to single-pass configurationas in FIG. 4. For instance, when filter 10 is operated at an attenuationdepth of 5 dB, the overall attenuation depth of double-pass filter 100becomes 10 dB.

Another embodiment of a device configuration for low polarizationdependence is shown in FIG. 24. In this embodiment, dual filter 110consists of filters 10′ and 10″ in tandem and connected through midfiber section 112. Filters 10′ and 10″ are preferably operated at thesame RF frequency. The birefringence of mid fiber section 112 isadjusted such that it acts as a half-wave plate aligned with 45-degreeangle with respect to the eigen polarization axes of filters 10′ and10″. In other words, the light passing through filter 10′ in one eigenpolarization state enters filter 10″ in the other eigen polarizationstate. If the polarization dependent loss is the same loss for filters10′and 10″, the overall attenuation after passing through filters 10′and 10″ becomes polarization-insensitive. If filters 10′ and 10″ are notidentical in terms of polarization dependent loss, the double filter 10would exhibit residual polarization dependent loss that should be,however, smaller than the polarization dependent loss of individualfilters, 10′ or 10″. Therefore, it is desirable that filters 10′ and 10″are identical devices. Since the filtering takes place by two filters,the drive powers to individual filters are reduced, compared to using asingle filter alone, to achieve the same attenuation depth.

In one embodiment, illustrated in FIG. 23, circulator 102 based onmagneto-optic crystal has overall insertion loss and polarizationdependent loss of 1.5 dB and 0.5 dB, respectively. Faraday rotatingmirror 104 has insertion loss and polarization dependent loss of 0.5 dBand 0.5 dB, respectively. Curve 120 in FIG. 24(a) shows the polarizationdependent loss profile in one embodiment when filter 10 was operated toproduce 10-dB attenuation at 1550 nm. The filter profile in thisinstance is shown by curve 124 in FIG. 24(b). Optical fiber 12 used inthe filter was a conventional communication-grade single mode fiber.When the filter was used in the double-pass configuration, the overallpolarization dependent loss was reduced greatly as shown by curve 121 inFIG. 24(a).

The polarization dependent loss was reduced down to less than 0.2 dB.The total insertion loss of double-pass filter was 3 dB, mainly due tothe circulator and splices. In this embodiment, the drive power tofilter 10 required to produce total 10-dB attenuation at 1550 nm, asshown by filter profile 125 in FIG. 24(b), was decreased compared to thesingle-pass filter experiment.

In another embodiment, illustrated in FIG. 25, two filters werefabricated with a conventional circular-core single mode fiber. Eachfilter was operated with 5-dB attenuation at the same center wavelength,1550 nm. The overall dual filter profile is shown by curve 126 in FIG.24(b). In these filters, the eigen polarization states are linear andtheir axes are parallel and orthogonal to the direction of the flexuralacoustic wave vibration or the acoustic polarization axis. This isgenerally true with filters made of a circular-core fiber where thedominant birefringence axes are determined by the lobe orientation ofthe cladding mode, which is the same as the acoustic polarization axis.Linear axis orthogonal to acoustic polarization is the slow axis, andits orthogonal axis is the fast axis. In this embodiment, a polarizationcontroller was used in mid fiber section 112 and controlled to minimizethe overall polarization dependent loss of dual filter 110.

The loss profile is shown by curve 122 in FIG. 24(a). The total filterprofile is shown by curve 126 in FIG. 24(b). The residual polarizationdependent loss as large as 0.6 dB is primarily due to differentpolarization dependent loss of filters 10′ and 10″, and could be reducedgreatly if identical two filters were used.

Another important characteristic of filter 10 is the intensitymodulation of an optical signal passing through the filter. One reasonwhich gives rise to the intensity modulation of the output signal isstatic coupling between the core and cladding modes either bymicrobending of fiber 12 or imperfect splices, if present. Anotherreason is an acoustic wave propagating backward in interactive region 36by an acoustic reflection at imperfect acoustic damper 30 and fiberjacket 32. FIG. 26 shows an example of output signal 139 suffering fromthe intensity modulation by backward acoustic reflection at acousticdamper 30. In this case, the major modulation frequency is equal totwice the acoustic frequency. The modulation depth is defined by theratio of peak-to-peak AC voltage amplitude, V_(AC) to DC voltage,V_(DC). By static mode coupling, the major modulation frequency is equalto the acoustic frequency. When both static mode coupling and backwardacoustic wave are present, the intensity of the output is modulated atfrequencies of both first- and second-harmonics of the acousticfrequency. The modulation depth, when smaller than 20%, is,approximately, linearly proportional to the amount of attenuation in dBscale. In most WDM communication system applications, the modulationdepth is generally required to be less than 3% at 10-dB attenuationlevel.

In one embodiment, illustrated in FIG. 4, filter 10 was fabricated byusing a conventional single-mode fiber. The modulation depth at 10-dBattenuation level was about 10% at both first- and second-harmonics ofthe acoustic frequency, as shown by curves 140 and 141 in FIG. 27(a),respectively. The same filter was used as filter 10 in anotherembodiment, illustrated in FIG. 23. The RF drive power to the filter wascontrolled to produce 10-dB attenuation depth. In the first embodimentof double-pass filter 100, the length of fiber section 106 was selectedsuch that the round-trip travel time of fiber section 106 is equal to aquarter of the period of the acoustic wave. In this case, thesecond-harmonics component of the intensity modulation can becompensated out. Curves 142 and 143 in FIG. 27(a) show the modulationdepth of first- and second-harmonics components, respectively. Thesecond-harmonics was eliminated almost completely. The first-harmonicswas also reduced a little, which may be attributed to imperfect lengthmatching of fiber section 106. In the second embodiment of double-passfilter 100, the length of fiber section 106 was such that the opticalround-trip travel time of fiber section 106 is equal to a half of theperiod of the acoustic wave. In this case, the first-harmonics componentof the intensity modulation can be reduced. Curves 147 and 148 in FIG.27(b) show the modulation depth of first- and second-harmonicscomponents, respectively. The first-harmonics was eliminated almostcompletely.

Reduction of intensity modulation can also be achieved by dual filter110 where the length of mid fiber section 112 is selected properly. Forexample, if the first-harmonic modulation component is to becompensated, the length of mid fiber section 112 is such that theoptical travel time from one end of section 112 to the other end isequal to a half of the period of the acoustic wave.

Referring now to FIG. 28, an optical add/drop multiplexer (OADM) 150includes filter 10 coupled to a first mode selective coupler 152 and asecond mode selective coupler 154. In this embodiment, filter 10 is afrequency selection AO mode converter. OADM 150 is reconfigurable inthat it is channel tunable. Multiple wavelengths, such as WDM signals,come into the input port. An added channel is added at the add port. Thesecond mode is added at the add port. Filter 10 is-frequency selectiveand cross-couples the frequency between the multiple waves and the addedchannel.

In FIG. 29, an OADM 156 includes a first filter 10′ and a second filter10″, both of which are frequency selection AO mode converters First modeselective coupler 152 is coupled to an output of first filter 10′ while.Second mode selective coupler 154 is coupled to an input of secondfilter 10″. At least one channel is dropped at first mode couple 152. Atleast one channel is added at second mode coupler 154.

Referring now to FIG. 30, multiple OADM's 150 and 156 ar utilized. EachOADM 150 or 156 is coupled to an optical fiber. Each optical fibercarries single or multiple channel bands. An input of multiple channelsand/or band is coupled to a splitter 158 and an output is coupled to acombiner 160. Splitter 158 and combiner 160 can be AWG's. Splitter 158splits channels and/or bands into multiple groups in the differentfibers. The spacing between adjacent channels and/or bands in the samegroup can be the original channel spacing (e.g., 100 GHz) times thenumber of groups.

In FIG. 31, the input is coupled to a first wavelength interleaver 162which separates odd channels on one fiber and even channels on anotherfiber. The fiber carrying the odd channels is coupled to a secondinterleaver 164 while the fiber carrying the even channels is coupled toa third interleaver 166. An OADM 150 or 156 is coupled to interleaver164 and 166. Additionally, OADM's 150 or 156 can be coupled tointerleaver 162.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. An add/drop multiplexer, comprising: a modeconverter including, an optical fiber with a longitudinal axis, a coreand a cladding in a surrounding relationship to the core, the opticalfiber having multiple spatial modes, an acoustic wave propagation memberwith a proximal end and a distal end, the distal end being coupled tothe optical fiber, the acoustic wave propagation member propagating anacoustic wave from the proximal to the distal end and launch an acousticwave in the optical fiber, at least one acoustic wave generator coupledto the proximal end of the acoustic wave propagation member; a firstmode selective coupler coupled to an input of the mode converter; and asecond mode selective coupler coupled to an output of the modeconverter.
 2. An add/drop multiplexer, comprising: a first modeconverter including, an optical fiber with a longitudinal axis, a coreand a cladding in a surrounding relationship to the core, the opticalfiber having multiple spatial modes, an acoustic wave propagation memberwith a proximal end and a distal end, the distal end being coupled tothe optical fiber, the acoustic wave propagation member propagating anacoustic wave from the proximal to the distal end and launch an acousticwave in the optical fiber, at least one acoustic wave generator coupledto the proximal end of the acoustic wave propagation member; a secondmode converter coupled to the optical fiber and including, an acousticwave propagation member with a proximal end and a distal end, the distalend being coupled to the optical fiber, the acoustic wave propagationmember propagating an acoustic wave from the proximal to the distal endand launch an acoustic wave in the optical fiber, at least one acousticwave generator coupled to the proximal end of the acoustic wavepropagation member; a first mode selective coupler coupled to an outputof the first mode converter; and a second mode selective coupler coupledto an output of the first mode converter and an input of the second modeconverter.
 3. The multiplexer of claim 1, wherein a wavelength of anoptical signal is coupled from one mode to another mode by varying asignal applied to the acoustic wave generator.
 4. The multiplexer ofclaim 1, wherein the acoustic wave generator produces multiple acousticsignals with individual controllable strengths and frequencies and eachof the acoustic signals provides a coupling between one mode to adifferent mode.
 5. The multiplexer of claim 1, wherein an amount of anoptical signal coupled from one mode to another mode is changed byvarying an amplitude of a signal applied to the acoustic wave generator.6. The multiplexer of claim 1, wherein the mode converter couples powerfrom at least one spatial mode into a different spatial mode.
 7. Themultiplexer of claim 1, wherein the mode converter couples power from atleast one core mode into a different core mode.
 8. The multiplexer ofclaim 1, wherein the mode converter couples power from at least one coremore into a cladding mode.
 9. The multiplexer of claim 1, wherein themode converter couples power from at least one cladding mode into a coremode.
 10. An add/drop multiplexer, comprising: a mode converterincluding an optical fiber with a longitudinal axis, a core and acladding in a surrounding relationship to the core, the optical fiberhaving multiple spatial modes; a first mode selective coupler coupled toan input of the mode converter; and a second mode selective couplercoupled to an output of the mode converter.
 11. The multiplexer of claim10, wherein a wavelength of an optical signal is coupled from one modeto another mode by varying a signal applied to the acoustic wavegenerator.
 12. The multiplexer of claim 10, wherein the mode converterproduces multiple acoustic signals with individual controllablefrequencies and each of the acoustic signals provides a coupling betweenone mode to a different mode.
 13. The multiplexer of claim 10, whereinan amount of an optical signal coupled from one mode to another mode ischanged by varying an amplitude of a signal applied to the modeconverter.
 14. The multiplexer of claim 10, wherein the mode convertercouples power from at least one spatial mode into a different spatialmode.
 15. The multiplexer of claim 10, wherein the mode convertercouples power from at least one core mode into a different core mode.16. The multiplexer of claim 10, wherein the mode converter couplespower from at least one core mode into a cladding mode.
 17. Themultiplexer of claim 10, wherein the mode converter couples power fromat least one cladding mode into a core mode.
 18. An add/drop apparatus,comprising: a splitter; a first add/drop multiplexer coupled to thesplitter and including a first mode converter; a second add/dropmultiplexer coupled to the splitter and including a second modeconverter; and a combiner coupled to the first and second add/dropmultiplexers.
 19. The apparatus of claim 18, wherein the splitter splitsone wavelength per fiber.
 20. The apparatus of claim 18, wherein thesplitter is a wavelength interleaver.
 21. The apparatus of claim 18,wherein the splitter includes a plurality of wavelength interleavers.22. The apparatus of claim 18, wherein the splitter is an arrayedwaveguide.
 23. The apparatus of claim 18, wherein the splitter splitsmultiple wavelengths to at least one fiber.
 24. The apparatus of claim18, wherein the combiner is an arrayed waveguide.
 25. The apparatus ofclaim 18, wherein the combiner combines multiple wavelengths to at leastone fiber.
 26. The apparatus of claim 18, wherein the first add/dropmultiplexer includes a first mode coupler coupled to the first modeconverter, and the second add/drop multiplexer includes a second modecoupler coupled to the second mode converter.
 27. The multiplexer ofclaim 2, wherein a wavelength of an optical signal is coupled from onemode to another mode by varying a signal applied to the acoustic wavegenerator.
 28. The multiplexer of claim 2, wherein the acoustic wavegenerator produces multiple acoustic signals with individualcontrollable strengths and frequencies and each of the acoustic signalsprovides a coupling between one mode to a different mode.
 29. Themultiplexer of claim 2, wherein an amount of an optical signal coupledfrom one mode to another mode is changed by varying an amplitude of asignal applied to the acoustic wave generator.
 30. The multiplexer ofclaim 2, wherein each mode converter couples power from at least onespatial mode into a different spatial mode.
 31. The multiplexer of claim2, wherein each mode converter couples power from at least one core modeinto a different core mode.
 32. The multiplexer of claim 2, wherein eachmode converter couples power from at least one core more into a claddingmode.
 33. The multiplexer of claim 2, wherein each mode convertercouples power from at least one cladding mode into a core mode.